with one’s back turned, on one’s back
He was lying on his back.
El viejo estaba de espaldas a mí.
The old man had his back to me.
Estaba tendido de espaldas en la playa.
He was lying on his back on the beach.
Y Sophie Tucker se caerá de espaldas, lo sé, al ver su nombre debajo del mío.
And Sophie Tucker will fall backward, I know, when she sees her name beneath mine.
— Chicago (2002)
Se desmayó y se cayó de espaldas.
She fainted and fell on her back.
Tom se apoyó de espaldas sobre el sofá.
Tom leaned back on the sofa.